Magnetic recording medium, magnetic storage and method for reproducing information from magnetic recording medium

ABSTRACT

A magnetic recording medium includes a first magnetic layer; and a second magnetic layer formed on the first magnetic layer. The first magnetic layer and the second magnetic layer make exchange coupling therebetween and also, have their magnetizing direction in anti-parallel to one another. A net residual area magnetization of the first magnetic layer and the second magnetic layer is expressed by the following formula: |Mr 1 ×t 1 −Mr 2 ×t 2 | where Mr 1  and Mr 2  denote respective residual magnetizations of the first magnetic layer and the second magnetic layer, and t 1  and t 2  denote respective film thicknesses of them; and the net area magnetization at a first temperature is larger than the net area magnetization at a second temperature lower than the first temperature.

This application is a Division of application Ser. No. 10/949,034, filedSep. 24, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium, a magneticstorage and a method for reproducing information from the magneticrecording medium, suitable for high density recording, and, inparticular, to a magnetic recording medium, a magnetic storage and amethod for reproducing information from the magnetic recording medium inwhich the magnetic recording medium is selectively heated, andrecording/reproduction is performed.

2. Description of the Related Art

Recently, high density recording has been rapidly promoted at a rate of100% per year. In an in-plane recording method which is a main trend, itis presumed that the limit of plane recording density is 100 Gb/in². Thereason therefor is that, in a high density recording range, for thepurpose of medium noise reduction, a size of a crystal grain which actsas a magnetization unit in a recording layer is reduced, and a zigzag ina magnetization transition zone which is a boundary between crystalgrains is reduced. However, when the size of the crystal grain isreduced, a volume of the magnetization unit is reduced, and therebyresidual magnetization is reduced due to thermal fluctuation. Thusthermal stability may deteriorate.

As a magnetic recording medium which satisfies both the medium noisereduction and the thermal stability, a magnetic recording medium(so-called synthetic ferrimagnetic medium) which has two magnetic layerswhich make exchange coupling therebetween antiferromagnetically has beenproposed (for example, see Japanese Laid-open Patent Application No.2001-056924). In this configuration, a substantial volume of a crystalgrain corresponds to a sum of the two magnetic layers making exchangecoupling, thereby thermal stability remarkably increases, and also, itbecomes possible to further reduce the medium noise since it becomespossible to achieve microscopic crystal grains.

SUMMARY OF THE INVENTION

However, in order to further improve the recording density, it isnecessary to further improve the S/N ratio and the thermal stabilityeven in the above-mentioned synthetic ferrimagnetic medium.

For example, as a method for improving the thermal stability,magnetocrystalline anisotropy is increased in a magnetic layer. However,when the magnetocrystalline anisotropy is increased, a coercive forceincreases, a head magnetic field in a head required for carrying outrecording increases, and thus, overwrite performance is deteriorated.Conventionally, such a problem is solved by searching for a magneticmaterial which has a high saturation magnetic flux density Bs appliedfor a recording head. However, it is very difficult to develop amaterial having a further higher Bs.

On the other hand, in a field of a magneto-optical recording, aso-called thermal assist recording method has been employed in which amagneto-optical recording medium is selectively heated, a temperaturethereof increases, and recording is performed on a part having thethus-reduced coercive force. In this method, a material having largemagnetocrystalline anisotropy can be employed, thus thermal stability isimproved, and also, recording is made possible with a relatively lowhead magnetic field. However, in this method, there are limits inincreasing a laser power and in reducing a spot size required forincreasing the recording density. Furthermore, there is a limit inreduction of the coercive force even thanks to temperature rise forachieving a high transfer rate. Accordingly, even in this method, it isdifficult to achieve further increasing in the recording density even bymeans of improvement of thermal stability only thanks to the increase inmagnetocrystalline anisotropy.

The present invention has been devised in consideration of theabove-mentioned problem, and an object of the present invention is toprovide a magnetic recording medium, a magnetic storage and a method forreproducing information from the magnetic recording medium by which boththe high S/N ratio and the improved thermal stability of written bitsare achieved, and the recording density is further increased.

According to a first aspect of the present invention, a magneticrecording medium is provided in which a first magnetic layer and asecond magnetic layer formed on the first magnetic layer are provided,the first magnetic layer and the second magnetic layer make exchangecoupling with one another, and also, in a condition in which no externalmagnetic field is applied, magnetization in the first magnetic layer andmagnetization in the second magnetic layer are anti-parallel to oneanother. Further, a net residual area magnetization of the firstmagnetic layer and the second magnetic layer is expressed by|Mr1×t1−Mr2×t2| where Mr1 and Mr2 denote respective ones of the residualmagnetization of the first magnetic layer and the second magnetic layer,and t1 and t2 denote respective film thicknesses. Furthermore, the netresidual area magnetization at a first temperature is larger than thenet area magnetization at a second temperature lower than the firsttemperature.

In this configuration, since the net residual area magnetization|Mr1×t1−Mr2×t2| of the first magnetic layer and the second magneticlayer at the first temperature higher than the second temperature islarger than the net area magnetization at the second temperature,reproduction output increases, and S/N ratio can thus be increased.Further, the net area magnetization at the second temperature can bereduced, and as a result, a demagnetizing field from an adjacent bit isreduced in a case of an in-plane magnetic recording medium. Also, ademagnetizing field can be reduced, in a case of a vertical magneticrecording medium, and thus the thermal stability for written bits can beimproved.

The first temperature is set higher than the second temperature, andshould be appropriately selected at which the net residual areamagnetization |Mr1×t1−Mr2×t2| increases thanks to materials of the firstmagnetic layer and/or the second magnetic layer, compositions thereof,or such. Also, the first temperature is set at which the residualmagnetization in both the first magnetic layer and the second magneticlayer does not vanish. It is preferable that the first temperature isselected from a temperature range lower than 400° C. in terms of heatdurability of a substrate, it is preferable that the first temperatureis lower than 200° C. in terms of crystallization in a case of employingan amorphous layer as a foundation layer, and, it is further preferablethat the first temperature is selected from a range lower than 150° C.Also, it is preferable that the first temperature is higher than 65° C.in terms of thermal stability of the first magnetic layer and/or thesecond magnetic layer.

Further, the second temperature is a temperature at which the magneticrecording medium is normally used, in other words, a room temperature,and, for example, is preferably selected from a range between 0° C. and65° C. However, the second temperature is not limited to this range,and, for example, the second temperature may be a cooled temperature ina case where the magnetic recording medium is used at a temperaturecooled to be lower than the room temperature, or is used in anenvironment cooled to be lower than the room temperature.

Furthermore, the more a ratio |Mr1×t1−Mr2×t2|/Hc between the netresidual area magnetization |Mr1×t1−Mr2×t2| and a coercive force Hc canbe reduced, the more a magnetization transition width can be reduced. Ina conventional magnetic recording medium, the more the net residual areamagnetization is reduced, the more the reproduction output decreases andthe S/N ratio decreases. However, according to the present invention,since it is possible to increase the new residual area magnetization byincreasing the temperature, such a problem does not occur, and it ispossible to improve resolution and also improve the S/N ratio.

In a case where the first magnetic layer is located at a position on theside of a substrate, a relationship of the residual area magnetizationbetween the first magnetic layer and the second magnetic layer may besuch that Mr2×t2>Mr1×t1. Thereby, it is possible to accurately recordinformation to the second magnetic layer, nearer to a magnetic head,corresponding to a recording magnetic field inverting position of themagnetic head, and thus, NLTS is improved in a case of applying thein-plane magnetic recording medium. In a case of applying the verticalmagnetic recording medium, since a recording magnetic field is appliedto the second magnetic layer than to the first magnetic layer in a moreconcentrated manner, it is possible to shorten a magnetizationtransition zone in the second magnetic field, whereby a line recordingdensity can be improved.

According to another aspect of the present invention, a magnetic storageis provided in which a magnetic recording medium having a recordinglayer containing crystalline magnetic grains, a heating unit selectivelyheating the magnetic recording medium, and a recording unit having amagnetic recording head. In this configuration, the heating unit heatsthe magnetic recording medium, and also, information is recorded to themagnetic recording medium with the use of the magnetic recording head.

In this configuration, the recording layer containing crystallinemagnetic grains of the magnetic recording medium is selectively heated,coercive force (in detail, dynamic coercive force expressed by theexpression (1) described later) in the recording layer is lowered.Thereby, even when the coercive force is higher than the conventionalmagnetic recording medium in a state of not being heated, it is possibleto lower the coercive force as a result of heating the recording layeras mentioned above, in to the magnetic recording medium according to thepresent invention. Thereby, it is not necessary to increase a recordingmagnetic field from the magnetic head even for enabling easy recording,while keeping recording performance such as superior overwriteperformance or such, and thus, it is possible to achieve the high S/Nratio. Furthermore, since it is possible to increase the coercive forceor the magnetocrystalline anisotropy constant while keeping the superiorrecording performance, it is possible to improve the thermal stability.As a result, it is possible to achieve the magnetic storage having thehigh S/N ratio and the superior thermal stability.

Further, a magnetic storage may be provided according to the presentinvention in which a first magnetic layer and a second magnetic layerformed on the first magnetic layer are provided. The above-mentionedfirst magnetic layer and the second magnetic layer make exchangecoupling with one another, and also, in a state in which no externalmagnetic field is applied, magnetization in the first magnetic layer andmagnetization in the second magnetic layer are anti-parallel to oneanother. Further, a heating unit heats the above-mentioned magneticrecording medium selectively, and a recording/reproduction unit having amagnetic recording head and a magnetic reproduction head are provided inthe magnetic storage. In this configuration, the heating unit heats themagnetic recording medium, and also, information is recorded on themagnetic recording medium with the use of the recording/reproductionunit.

In this configuration, the magnetic recording medium having recordinglayers including the first magnetic layer and the second magnetic layermaking exchange coupling with one another antiferromagnetically isselectively heated, a coercive force (in detail, dynamic coercive forceexpressed by the expression (1) described later) in the recording layersis lowered. Thereby, even when the coercive force is higher than that inthe conventional magnetic recording medium in a state of not heating inthe magnetic recording medium according to the present invention, it ispossible to lower the coercive force as a result of heating therecording layers as mentioned above. Thereby, it is not necessary toincrease a recording magnetic field from the magnetic head even forenabling easy recording while keeping recording performance such assuperior overwrite performance or such, and thus, it is possible toachieve high S/N ratio. Furthermore, since it is possible to increasethe coercive force or magnetocrystalline anisotropy constant whilekeeping superior recording performance, it is possible to improvethermal stability. As a result, it is possible to achieve a magneticstorage having a high S/N ratio and superior thermal stability.

Further, in the magnetic storage according to the present invention, itis possible to keep overwrite performance and resolution even when arecording current supplied to the magnetic recording head is reduced incomparison to a case of applying a conventional magnetic recordingmedium. By thus reducing the recording current, it is possible toachieve a well-controlled distribution of a recording magnetic fieldfrom the magnetic recording head, and thus, it is possible toconcentrate the recording magnetic field at a desired track of themagnetic recording medium. Therefore, it is possible to remarkablyreduce side erase, avoid increase in the magnetization transition zoneotherwise occurring due to the recording magnetic field widely spreadingin a plane direction of the magnetic recording medium from the magneticrecording head when the recording current is on the order of 40 mAconventionally.

The temperature at which the magnetic recording medium is heated shouldbe such as that at which the coercive forces in the first magnetic layerand the second magnetic layer may decrease from a condition of notheated, and may be the above-mentioned first temperature. Thistemperature may be such as that at which the amount of exchange couplingbetween the first magnetic layer and the second magnetic layer, forexample, an exchange magnetic field may decrease. By thus weakening theexchange magnetic field between the first magnetic layer and the secondmagnetic layer, it becomes easier to switch a recording magnetic fielddirection so as to switch a magnetization direction in the firstmagnetic layer and/or the second magnetic layer, and it is possible toimprove the overwrite performance, resolution and also NLTS (non-lineartransition shift) performance.

Further, since a track width to record is determined by a width of arecording magnetic field applied by the magnetic recording head,specifically, a core width of the magnetic recording head, it ispossible to widen a zone to heat than the track width according to thepresent invention in comparison to a case of a conventionalmagneto-optical recording method, and it is possible to achieve anincreased track density more easily.

According to another aspect of the present invention, a magnetic storageis provided which includes a magnetic recording medium having a firstmagnetic layer and a second magnetic layer formed on the first magneticlayer, the first magnetic layer and the second magnetic layer makeexchange coupling therebetween, and also, magnetization in the firstmagnetic layer and magnetization in the second magnetic layer areanti-parallel to one another in a state in which no external magneticfield is applied. Further, a heating unit heating the magnetic recordingmedium selectively and a recording/reproduction unit are provided in themagnetic storage. In the magnetic storage, the heating unit heats themagnetic recording medium so as to increase reproduction output, andinformation recorded in the magnetic recording medium is reproduced bymeans of the recording/reproduction unit.

According to the present invention, a portion at which desiredinformation is recorded in the magnetic recording medium havingrecording layers including the first magnetic layer and the secondmagnetic layer making antiferromagnetically exchange couplingtherebetween is selectively heated, and thus, reproduction output beingincreased, whereby the S/N ratio can be improved. For a portion of notheated or in a case where heating is not performed, a state in which thereproduction output is low is created, for example, net areamagnetization in the first magnetic layer and the second magnetic layercan be reduced there or in such a case. As a result, in a case where anin-plane magnetic recording medium is applied, it is possible to reducea demagnetizing field from an adjacent bit, while, in a case of avertical magnetic recording medium, a demagnetizing field can be reducedand thus thermal stability in written bits can be improved.

FIGS. 1A and 1B partially show an in-plane magnetic recording medium forillustrating a principle of the present invention.

As shown, recording layers of the in-plane magnetic recording medium 10according to the present invention include a first magnetic layer 11, asecond magnetic layer 12 and a non-magnetic coupling layer 13 formedbetween the first and second magnetic layers 11 and 12. The first andsecond magnetic layers 11 and 12 are controlled by a film thickness ofthe non-magnetic coupling layer 13 or such so as to make exchangecoupling antiferromagnetically. In this configuration, in a state inwhich no external magnetic field is applied, magnetization in the firstmagnetic layer 11 and magnetization in the second magnetic layer 12 areoriented in anti-parallel directions with one another. In this case, amagnetic leakage field from the first and second magnetic layers 11 and12, i.e., the magnetic leakage field Hx used when a magnetic headreproduces information from the magnetic recording medium is inproportion to a net residual area magnetization |Mr1×t1−Mr2×t2| of thefirst and second magnetic layers 11 and 12 where Mr1 and Mr2 denoterespective ones of residual magnetization in the first magnetic layer 11and magnetization in the second magnetic layer 12, and t1 and t2 denoterespective film thicknesses.

According to the present invention, magnetic layers having differenttemperature characteristics for the residual magnetization are used incombination as the first magnetic layer 11 and the second magnetic layer12. A case where the temperature characteristics for the residualmagnetization may be, in terms of physicality, a case where Curietemperatures in ferromagnetic materials or compensation temperatures inferrimagnetic materials are different. In terms of materials, it may bea case where sizes of crystal grains are different. This case may be acase where the first magnetic layer 11 and the second magnetic layer 12are made from many crystal grains or a case where they are made fromamorphous materials. There, description will be made assuming that theCurie temperature of the first magnetic layer 11 is lower than the sameof the second magnetic layer 12. In other words, as a result of beingheated, a reduction rate in the residual magnetization is larger in thefirst magnetic layer 11 than in the second magnetic layer 12.

FIG. 1A shows a magnetization state in a case where the temperature inthe magnetic recording medium is around a room temperature (i.e., oneexample of the second temperature claimed in claim 1), while FIG. 1Bshows a magnetization state in which the temperature in the magneticrecording medium is higher than the room temperature, for example, 100°C. (one example of the first temperature claimed in claim 1). Theresidual area magnetization is such that Mr2×t2>Mr1×t1 around the roomtemperature and also at 100° C. Since Mr1 at 100° C. decreases at areduction rate higher than that of Mr2, the net residual areamagnetization at 100° C. becomes larger than the net residual areamagnetization around the room temperature. That is, in the case of FIG.1A around the room temperature, the residual area magnetization in thefirst mutagenic layer 11 and the residual area magnetization in thesecond magnetic layer 12 are approximately same as one another.Accordingly, magnetic leakage fields from the first and second magneticlayers 11 and 12 cancel out by one another, so as to become smaller. Incontrast thereto, in the case of FIG. 1B at 100° C., since the netresidual area magnetization in the first magnetic layer 11 decreases, anamount of canceling out between the respective magnetic leakage fieldsfrom the first and second magnetic layers 11 and 12 decreases. As aresult, a magnetic leakage field generated from the first and secondmagnetic layers 11 and 12 becomes larger than that in the case aroundthe room temperature. Accordingly, the magnetic filed from the first andsecond magnetic layers 11 and 12 at 100° C., which the reproductionmagnetic head can detect, increases, and as a result, reproductionoutput increases. Thus, in comparison to a conventional syntheticferrimagnetic medium, reproduction output can be increased, and as aresult, S/N ratio can be improved.

On the other hand, at the temperature around the room temperature, inthe magnetic recording medium according to the present invention, as aresult of setting the residual area magnetization in the first magneticlayer 11 and the residual area magnetization in the second magneticlayer 12 to be approximately same as one another in this condition, orto have a predetermined slight difference therebetween, since themagnetization in the first magnetic layer 11 and the magnetization inthe second magnetic layer 12 are anti-parallel to one another, the netresidual area magnetization in the first and second magnetic layers 11and 12 are canceled out by one another. Accordingly, the magneticleakage field generated from the first and second magnetic layers 11 and12 are reduced. As a result, it is possible to reduce a demagnetizingfield applied to an adjacent bit (magnetic domain) each other. More thedemagnetizing fields are reduced, more the reduction in the residualmagnetization due to aging can be reduced well, thermal stability inwritten bits is improved, and thus, it is possible to provide a magneticrecording medium having superior thermal stability.

The principle of the present invention is described next in more detail.

FIGS. 2A and 2B show temperature characteristics for the residual areamagnetization in the magnetic recording medium according to the presentinvention.

With reference to FIG. 2A, in the magnetic recording medium according tothe present invention, a relationship of residual area magnetization inthe amounts and the Curie temperatures Tc1 and Tc2 between the firstmagnetic layer 11 and the second magnetic layer 12 becomes the same asthat shown in FIGS. 1A and 1B. In other words, at a temperature T1, theresidual area magnetization in the second magnetic layer 12 is largerthan that in the first magnetic layer 11, and the first magnetic layer11 and the second magnetic layer 12 have the net residual areamagnetization A. Since the first magnetic layer 11 has the lower Curietemperature than that of the second magnetic layer 12 (Tc1<Tc2), whenthis magnetic recording medium is heated to the temperature T2 aroundthe Curie temperature Tc1, the reduction rate in the residual areamagnetization in the first magnetic layer 11 is so large that the netresidual area magnetization B at this case increases remarkably from thenet residual area magnetization A at the temperature T1. Accordingly,since reproduction output is in proportion to the net residual areamagnetization B, it is possible to remarkably increase the reproductionoutput as a result of heating to the temperature T2.

With reference to FIG. 2B, according to a magnetic recording medium inanother example of the present invention, a relationship in the Curietemperature between the first magnetic layer 11 and the second magneticlayer 12 is different from that shown in FIG. 2A. That is, the Curietemperature of the second magnetic layer 12 is lower than that of thefirst magnetic layer 11 (Tc1>Tc2). In other words, although thismagnetic recording medium has a net residual area magnetization C whichis approximately same as that of FIG. 2A at a temperature T1, theresidual area magnetization of the second magnetic layer 12 decreaseswhen it is heated, thereby it becomes approximately same as that of thefirst magnetic layer 11, and then, after it is further heated to atemperature T2, the residual area magnetization of the second magneticlayer 12 becomes smaller than that of the first magnetic layer 11. FIG.2B also shows a magnetic leakage field of the first and second magneticlayers 11 and 12 in this case. As shown, the magnetic leakage field inproportion to the net residual area magnetization D has an orientationwhich is opposite to that at the temperature T1. Consequently, it can beseen that, also in this case, the same as in the case of FIG. 2A, it ispossible to remarkably increase the reproduction output by heating themagnetic recording medium to the temperature T2.

Furthermore, not only for the in-plane magnetic recording medium asdescribed above, the present invention can also be applied for avertical magnetic recording medium, as is described next.

FIGS. 3A and 3B partially show a vertical magnetic recording medium forillustrating a principle of the present invention. As shown, thevertical magnetic recording medium 14 according to the present inventionhas recording layers including a first magnetic layer 15, a secondmagnetic layer 16, and a non-magnetic coupling layer 13 formedtherebetween. The first and second magnetic layers 15 and 16 arecontrolled by a film thickness of the non-magnetic coupling layer 13 orsuch and make antiferromagnetically exchange coupling therebetween, and,in a state in which no external magnetic field is applied, magnetizationin the first magnetic layer 15 and magnetization in the second magneticlayer 16 are oriented perpendicular to the film surface in anti-parallelto one another. In this case, a magnetic field leaking from the firstand second magnetic layers 15 and 16, i.e., a magnetic field Hy which isused for reproducing information from the magnetic recording medium 14by means of a magnetic head, is in proportion to the net residual areamagnetization |Mr1×t1−Mr2×t2| of the first and second magnetic layers 15and 16 where Mr1 and Mr2 denote respective ones of residualmagnetization in the first magnetic layer 15 and residual magnetizationin the second magnetic layer 16, and t1 and t2 denote respective filmthicknesses. In a case where the Curie temperature of the first magneticlayer 15 is lower than that of the second magnetic layer 16, a reductionrate in the residual magnetization in the first magnetic layer 15becomes larger than that in the second magnetic layer 16 when themagnetic recording medium 14 is heated. As a result, for example, at atemperature around the Curie temperature of the first magnetic layer 15,Mr1 decreases at the reduction rate larger than that of Mr2, and as aresult, the net residual area magnetization increases to become largerthan the same around the room temperature. Accordingly, reproductionoutput increases, and thus, S/N ratio is improved.

Thus, according to the present invention, by performing recording in astate in which coercive force is lowered as a result of a magneticrecording medium being heated at a time of recording, overwriteperformance is improved, recording density is increased, and also, it ispossible to provide both high S/N ratio and superior thermal stability.Furthermore, according to the present invention, a net areamagnetization in a magnetic recording medium is reduced around a roomtemperature in comparison to a case of a conventional magnetic recordingmedium, while the same increases as the temperature is raised.Accordingly, reproduction output is increased and the S/N ratio isimproved upon at a time or reproduction as a result of the magneticrecording medium being heated, while the thermal stability is improvedsince information recorded can be positively maintained at the roomtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and further features of the present invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings:

FIGS. 1A and 1B typically show a part of an in-plane magnetic recordingmedium for illustrating a principle of the present invention;

FIGS. 2A and 2B shows temperature characteristics of residual areamagnetization in a magnetic recording medium according to the presentinvention;

FIGS. 3A and 3B typically show a part of a vertical magnetic recordingmedium for illustrating a principle of the present invention;

FIG. 4 shows an elevational sectional view of an in-plane magneticrecording medium according to a first embodiment of the presentinvention;

FIG. 5 shows temperature characteristics of residual magnetization in amagnetic disk in a reference example;

FIG. 6 shows temperature characteristics of residual area magnetizationin a magnetic disk in a first embodiment of the present invention;

FIG. 7 shows an elevational sectional view of a vertical magneticrecording medium according to a second embodiment of the presentinvention;

FIG. 8 shows an elevational sectional view of a patterned mediumaccording to a third embodiment of the present invention;

FIG. 9 shows an elevational sectional view of a patterned mediumaccording to a first variant embodiment of the third embodiment of thepresent invention;

FIG. 10 shows an elevational sectional view of a patterned mediumaccording to a second variant embodiment of the third embodiment of thepresent invention;

FIG. 11 partially shows a plan view of a magnetic storage according to afourth embodiment of the present invention;

FIG. 12 partially shows an elevational sectional view of the magneticstorage according to the fourth embodiment of the present invention;

FIG. 13 shows magnetic characteristics of a magnetic disk 1 and amagnetic disk 2;

FIG. 14 shows thermal stability of the magnetic disk 1 and the magneticdisk 2;

FIGS. 15A and 15B show relationship between overwrite performance andlaser output in the first and second magnetic disks, respectively;

FIG. 16 shows solitary wave half-value width characteristics of themagnetic disk 1 and the magnetic disk 2;

FIG. 17 shows a relationship between S/N ratio and laser output in themagnetic disk 1;

FIG. 18 shows a relationship between laser output and recording currentin which an S/N ratio becomes maximum.

FIG. 19 shows a relationship between a generated magnetic field and arecording current in a recording device of the magnetic storage;

FIG. 20 shows an average output and a recording current in a lowrecording density in the magnetic disk 1; and

FIG. 21 shows change in normalized average output at a time of laserbeam application in the magnetic disk 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention is described now.

FIG. 4 shows an elevational sectional view of an in-plane magneticrecording medium according to a first embodiment of the presentinvention. As shown, the in-plane magnetic recording medium according tothe first embodiment includes a substrate 21, and, thereon, a first seedlayer 22, a second seed layer 23, a foundation layer 24, a non-magneticintermediate layer 25, a first magnetic layer 26, a non-magneticcoupling layer 28, a second magnetic layer 29, a protective layer 30 anda lubrication layer 31 are formed in the stated order. The in-planemagnetic recording medium 20 has an exchange coupling structure in whichthe first magnetic layer 26 and the second magnetic layer 29 makeantiferromagnetically exchange coupling therebetween via thenon-magnetic coupling layer 28. There, magnetization oriented in anin-plane direction in the first magnetic layer 26 and the secondmagnetic layer 29 is oriented in anti-parallel between the first andsecond magnetic layers 26 and 29 in a condition in which no externalmagnetic field is applied. Further, as described above for the principleof the present invention, the first magnetic layer 26 and the secondmagnetic layer 29 have temperature characteristics different from oneanother for magnetization or residual magnetization (hereinafter,‘magnetization or residual magnetization’ is simply referred to as‘residual magnetization’ unless otherwise stated), and for example, havedifferent Curie temperatures or compensation temperatures (hereinafter,unless otherwise stated, ‘Curie temperatures or compensationtemperature’ are simply referred to as ‘Curie temperatures’).Furthermore, there may be a case where, even when they haveapproximately the same Curie temperatures, they have different spinarrangements, that is, for example, one has ferrimagnetism and the otherhas ferromagnetism, or such.

As the substrate 21, for example, a disk-shaped plastic substrate, glasssubstrate, a NiP-plated aluminum alloy substrate, silicon substrate orsuch may be applied. In particular, in a case where the substrate 21 isa tape-shaped one, a plastic film such as PET, PEN, polyimide or suchmay be applied. The substrate 21 may have texture treatment performedthereon or not performed thereon. The texture treatment is performed ina circumferential direction, i.e., in a track longitudinal direction, ina case where the in-plane magnetic recording medium 20 is a magneticdisk.

The first seed layer 22 is made of non-magnetic material, for example,NiP, CoW, CrTi or such, and may have texture treatment performed thereonor not performed thereon. It is preferable that oxidization treatment isperformed when the first seed layer 22 is made of amorphous materialsuch as NiP or such. Thereby, in-plane orientation along c-axis isimproved in the magnetic layer 26 and magnetic layer 29. Further, it ispossible to employ a known material improving c-axis orientation insteadof NiP.

The second seed layer 23 is made of, for example, amorphous materialsuch as NiP, CoW, CrTi or such, or an alloy having a B2 structure suchas AlRu, NiAl, FeAl or such. In a case where the second seed layer 23 ismade of amorphous material and the foundation layer 24 formed thereaboveis made of an alloy having a B2 structure, orientation in a (001) planeor a (112) plane is improved. Texture treatment may be performed or not.The texture treatment should be performed in a circumferentialdirection, i.e., a track longitudinal direction in a case where thein-plane magnetic recording medium 20 is a magnetic disk.

The foundation layer 24 is made of, for example, Cr, a Cr alloy such asCrMo, CrW, CrV, CrB, CrMoB or such, or an alloy having a B2 structuresuch as AlRu, NiAl, FeAl or such. As described above, the foundationlayer 24 is formed by epitaxial growth on the second seed layer 23, and,when the foundation layer 24 has a B2 structure, a (001) plane or a(112) plane exhibits satisfactory orientation in the growth direction.In a case where the foundation layer 24 is made of Cr or a Cr alloy, a(002) plane exhibits satisfactory orientation in the growth direction.The foundation layer 24 may be made of a plurality of laminated layersmade from a Cr alloy or an alloy having a B2 structure. By applyinglamination of a plurality of layers, orientation in the foundation layer24 itself is improved, epitaxial growth of the non-magnetic intermediatelayer 25 is made satisfactory, and further, orientation in the firstmagnetic layer 26 and the second magnetic layer 29 can be improved.

The non-magnetic intermediate layer 25 is made from, for example, anon-magnetic alloy having an hcp structure in which an element or alloyM is added to a CoCr alloy, and, a film thickness is set in a rangebetween 1 nm and 5 nm. The above-mentioned ‘M’ denotes one selected fromamong Pt, B, Mo, Nb, Ta, W, Cu and an alloy thereof. The non-magneticintermediate layer 25 is formed by epitaxial growth succeedingcrystallinity and crystal grain size to the foundation layer 24,improves the crystallinity of the first magnetic layer 26 and the secondmagnetic layer 29 which are then formed by epitaxial growth on thenon-magnetic intermediate layer 25, reduces a distribution range ofcrystal grain (magnetic grain) sizes, and promotes c-axis orientation inthe in-plane direction (in a direction parallel to the substrate plane).Further the non-magnetic intermediate layer 25 may be made of aplurality of laminated layers made from the above-mentioned alloy.Thereby, orientation in the first magnetic layer 26 and the secondmagnetic layer 29 can be improved.

A grating constant of the non-magnetic intermediate layer 25 may be madedifferent by several percent from that of the first magnetic layer 26 orthe second magnetic layer 29, and inner stress may be made to occur inthe in-plane direction in an interface between the non-magneticintermediate layer 25 and the first magnetic layer 26 or inside thefirst magnetic layer 26. Thereby, it is possible to increase staticcoercive force in the first magnetic layer 26. Further, the non-magneticintermediate layer 25 may be provided or may not be provided.

A film thickness of the first magnetic layer 26 may be set in a rangebetween 0.5 nm and 20 nm, and is made of Co, Ni, Fe, a Co alloy, a Nialloy, a Fe alloy or such. In particular, it is preferable to employmaterial such as Co, CoCr, CoCrTa, CoPt, CoCrPt or such, or, at leastone of a group of rare-earth elements including Gd, Tb, Dy, Pr, Nd, Yb,Sm, Ho and Er is preferably added thereto. In such an alloy, polycrystalin which crystal grains are separated by grain boundaries is produced,spins which Co atoms and Gd atoms in crystal grains have, have aferrimagnetic arrangement, and thus, become anti-parallel to oneanother. By controlling an amount of such a rare-earth to add, it ispossible to control temperature characteristics of residualmagnetization. Thereby, it is possible to increase a reduction rate ofresidual magnetization in the first magnetic layer 26 with respect to atemperature larger than that in the second magnetic layer 29 describedlater. It is possible to control the temperature characteristics ofresidual magnetization in the first magnetic layer 26 also by reducingthe crystal grain diameter, reducing anisotropic magnetization, or such.Furthermore, it is possible to control the temperature characteristicsof residual magnetization also by making composition of the firstmagnetic layer 26 slightly different from that of the second magneticlayer 29. Furthermore, the temperature characteristics of residualmagnetization may be controlled also as a result of phase transitiontemperature inherent to the material such as Curie temperature beingadjusted.

The first magnetic layer 26 is formed by epitaxial growth on thenon-magnetic intermediate layer 25, the c-axis thereof is oriented inthe in-plane direction, and the direction of easy axis of magnetizationbecomes the in-plane direction. It is preferable to add materialselected from among B, Mo, Nb, Ta, W, Cu and an alloy thereof to theabove-described material. Thereby, it is possible to control the crystalgrain diameter. Furthermore, the first magnetic layer 26 may include aplurality of layers laminated. Thereby, orientation in the secondmagnetic layer 29 can be improved.

The non-magnetic coupling layer 28 is made of, for example, Ru, Rh, Ir,a Ru alloy, an Rh alloy, an Ir alloy or such. Thereamong, Rh and Ir havean fcc structure, while Ru has an hcp structure, and Ru has a gratingconstant such as a=0.27 nm which is approximate to that of a CoCrPtalloy having a grating constant of a=0.25 nm. Therefore, Ru or a Rualloy is suitable to be employed. As the Ru alloy, an alloy of any oneof Co, Cr, Fe, Ni and Mn, or an alloy thereof, with Ru, is preferable.

A film thickness of the non-magnetic coupling layer 28 is set in a rangebetween 0.4 nm and 1.5 nm (preferably in a range between 0.6 nm and 0.9nm, or, in a case of the Ru alloy, a range between 0.8 nm and 1.4 nm ispreferable although it depends on the content of Ru therein). The firstmagnetic layer 26 and the second magnetic layer 29 make exchangecoupling therebetween with the non-magnetic coupling layer 28 insertedtherein. By setting the film thinness of the non-magnetic coupling layer28 in the above-mentioned range, magnetization in the first magneticlayer 26 and magnetization in the second magnetic layer 29 coupleantiferromagnetically with one another, and, as shown in FIG. 4, theybecome anti-parallel to one another in a condition in which no externalmagnetic field is applied. In particular, it is preferable to determinethe film thickness of the non-magnetic coupling layer 28 to so as tocorrespond to the first antiferromagnetic peak (a peak on the side ofthe thinnest film thickness) of vibration-type exchange couplingdepending on the thickness of the non-magnetic coupling layer.

A film thickens of the second magnetic layer 29 is set in a rangebetween 5 nm and 20 nm, and is made from Co, Ni, Fe, a Co alloy, a Nialloy, a Fe alloy or such. Especially, CoPt, CoCrTa or CoCrPt, or,material obtained from adding B, Mo, Nb, Ta, W, Cu or an alloy theretois preferable. The second magnetic layer 29 is set to have temperaturecharacteristics different from those of the first magnetic layer 26 asdescribed above. Further, the same as in the first magnetic layer 26,material obtained from adding at least one element selected from among arare-earth element group including Gd, Tb, Dy, Pr, Nd, Yb, Sm, Ho and Ermay be used as the material of the second magnetic layer 29. Further,the second magnetic layer 29 may be made of a plurality of layerslaminated.

In a relationship between the first and second magnetic layers 26 and29, it is preferable to set them so that Mr1×t1<Mr2×t2 holds where Mr1and Mr2 denote residual magnetization in the first magnetic layer 26 andresidual magnetization in the second magnetic layer 29, and t1 and t2denote respective film thicknesses. Thereby, the second magnetic layer29 has magnetization in the same direction as that of the net residualarea magnetization, and it is possible to accurately record informationto the second magnetic layer 29 corresponding to a position at which arecording magnetic field of the magnetic head is inverted. It is alsopossible that a setting is made such that Mr1×t1>Mr2×t2. As a result ofthe first magnetic layer 26 and the second magnetic layer 29 being madeto be thin films, the above-mentioned problem otherwise occurring at atime of recording is solved.

In a case where the setting is made such that Mr1×t1<Mr2×t2, therespective Curie temperatures Tc1 and Tc2 of the first and secondmagnetic layers 26 and 29 may be such that Tc1<Tc2 or Tc1>Tc2 asdescribed above for the principle of the present invention. Thissituation is same also in the case of setting of Mr1×t1>Mr2×t2. Sincethe in-plane magnetic recording medium 20 is used or stored normally ata room temperature, Tc1 and Tc2 should be higher than the roomtemperature.

By setting the composition of the first magnetic layer 26 or the secondmagnetic layer 29 so that the compensation temperature may be lower thanthe room temperature, it is possible to increase the residualmagnetization in the first magnetic layer 26 or the second magneticlayer 29 by heating, as a result of appropriately selecting the heatingtemperature at this time, and thus, it is possible to increase the netresidual area magnetization.

Further, it is also possible to make a setting such that the residualarea magnetization of the first magnetic layer 26 and the secondmagnetic layer 29 may become approximately such that Mr1×t1=Mr2×t2around the room temperature. Thereby, it is possible to remarkablyreduce a demagnetizing field from an adjunct bit, and thus, it ispossible to greatly improve thermal stability of written bits. In thiscase, it is preferable that a servo signal is previously recorded suchas in a servo build-in type medium. Thereby, even when at a time of nonheating condition, access to a target track is enabled. However, it isalso possible to make such a difference between Mr1×t1 and Mr2×t2 that aservo signal recorded in the in-plain magnetic recording medium 20 at alow recording line density may be reproduced without heating. Forexample, a setting may be made such that the net residual areamagnetization |Mr1×t1−Mr2×t2| may lie in a range between 1.26 nTm and5.02 nTm (0.1 memu/cm² and 0.4 memu/cm²) so that, around the roomtemperature, output may be reduced by 10% through 80% from output of theconventional in-plane magnetic recording medium.

A film thickness of the above-mentioned protective layer 30 may be setin a range between 0.5 nm and 10 nm (preferably, in a range between 0.5nm and 5 nm), and, for example, is made of a diamond like carbon, carbonnitride, amorphous carbon or such.

The above-mentioned lubrication layer 31 is made from an organic liquidlubricant having perfluoropolyether as a main chain with a terminalgroup of —OH, a benzene ring or such. Specifically, the lubricationlayer 31 may have a thickness in a range between 0.5 nm and 3.0 nm, and,ZDol (provided by Monte Fluos Co. Ltd., with the terminal group of —OH),AM3001 (provided by AUSIMONT KK, with the terminal group of benzenering), Z25 (provided by Monte Fluos Co. Ltd.) or such may be applied.The lubricant is appropriately selected to be suitable for the materialof the protective layer 30. The above-mentioned respective layers exceptthe lubrication layer 31 are produced by a sputtering method, a vacuumdeposition method or such. The lubrication layer 31 is produced by a dipcoating method, a spin coater method or such, and, in a case where thein-plane magnetic recording medium 20 is a tape-like one, a die coatingmethod or such may be applied.

According to the first embodiment, as a result of the first magneticlayer 26 and the second magnetic layer 29 making antiferromagneticallyexchange coupling therebetween being made to have respective temperaturechanges different from one another, it is possible to obtain the netresidual area magnetization which is larger than that around the roomtemperature by heating them. Thereby, it is possible to increasereproduction output. Accordingly, it is possible to improve the S/Nratio remarkably from that in the conventional in-plane magneticrecording medium. Furthermore, around the room temperature, it ispossible to reduce the net area magnetization, and thus, it is possibleto control a demagnetizing field from an adjunct bit so as to improvethe thermal stability of written bits.

Furthermore, in the in-plane magnetic recording medium 20 in the firstembodiment, also in a recording process, the same as in the reproductionprocess, a portion at which recording is made may be heated at a time ofapplying a recording magnetic field. Thereby, a coercive force in thefirst magnetic layer 26 and the second magnetic layer 29 decreases, andthus, it is possible to reduce a recording magnetic field required forswitching a magnetization direction. Further, since residualmagnetization of each of the first and second magnetic layers 26 and 29is reduced from that around the room temperature, and exchange couplingeffect therebetween also decreases, as a result an exchange magneticfield applied to one another decreases, thereby carrying out of rotationof magnetization direction is made easier, and thus, overwriteperformance is further improved.

A specific example of the first embodiment of the present invention isdescribed next. First, a magnetic disk in a reference example wasproduced for measuring temperature change in residual magnetization ofthe first magnetic layer. The magnetic disk in this reference examplehad a specific configuration of glass substrate/CrTi layer (with athickness of 25 nm)/AlRu layer (with a thickness of 15 nm)/CrMo layer(with a thickness of 5 nm)/CoCrTa layer (with a thickness of 1nm)/CoCrTaGd layer as the first magnetic layer (with a thickness of 10nm)/diamond like carbon (DLC) layer (with a thickness of 4.0 nm). TheCoCrTaGd layer has a specific configuration of(Co₈₂Cr₁₃Ta₅)_(100-x)Gd_(x), where x=0, 8 or 16 atomic %. Each of thenumeric values in the composition is expressed in atomic %. A DCmagneto-sputtering apparatus was used for producing these films.

FIG. 5 shows temperature characteristics in the magnetic disk in thereference example. As shown, it can be seen that, with respect to a caseof x=0 atomic %, the residual magnetization at 400 K remarkablydecreases from that at 300 K in a case of x=8 atomic %. In other words,it can be seen that, by using a magnetic layer having the composition ofx=8 atomic % as the first magnetic layer with selecting the thicknessesof the first magnetic layer and the second magnetic layer so that thenet residual area magnetization at 300 K may lie within a desired range,it is possible to configure the in-plane magnetic recording medium 20according to the first embodiment of the present invention in which theheating temperature is set as 400 K, for example.

Further, in the composition of x=16 atomic %, the residual magnetizationbecomes 0 (compensation temperature) around 270 K, and the residualmagnetization increases at 350 K from 270 K. By using a magnetic layerhaving the composition of x=0 atomic % as the first magnetic layer whilea magnetic layer having the composition of x=16 atomic % as the secondmagnetic layer, it is possible to increase the net residual areamagnetization by heating it to 350 K for example. That is, it can beseen that, by using a magnetic layer having ferrimagnetism having thecompensation temperature below the room temperature, it is possible toincrease the residual magnetization and to increase the net areamagnetization by heating it to an appropriate temperature around such atemperature above the compensation temperature and in which the residualmagnetization may become maximum. The temperature measurement for theresidual magnetization was carried out with the use of a SQUIDapparatus.

As a magnetic disk as a specific example of according to the firstembodiment of the present invention, the following configuration wasproduced, in the above-described magnetic disk in the reference example;glass substrate/CrTi layer (with a thickness of 25 nm)/AlRu layer (witha thickness of 15 nm)/CrMo layer (with a thickness of 5 nm)/CoCrTa layer(with a thickness of 1 nm)/CoCrTaGd layer ((Co₈₁Cr₁₄Ta₅)₉₂Gd₈, with athickness of 2 nm)/Ru layer (with a thickness of 0.7 nm)/CoCrPtB layer(CoCrPt₁₄B₅ layer, with a thickness of 12 nm)/DLC layer (with athickness of 4.0 nm)/lubrication layer (AM3001, with a thickness of 1.2nm).

FIG. 6 shows temperature characteristics of residual area magnetizationof the above-mentioned magnetic disk in the specific example accordingto the first embodiment of the present invention. As shown, the residualarea magnetization in the CoCrTaGd layer sharply decreases from around350 K, while change in the residual area magnetization of the CoCrPtBlayer is small. Therefrom, it can be seen that the net residual areamagnetization sharply increases from around 350 K. Accordingly, byselecting the heating temperature from a range between 350 K and 400 K,it is possible to obtain high reproduction output and high S/N ratio.The data of the residual area magnetization of each of the CoCrTaGdlayer and the CoCrPtB layer was obtained from measurement carried outindependently on the magnetic disk having the above-mentionedconfiguration.

The number of recording layers is not limited to the two in the firstembodiment of the present invention, and, the recording layers mayinclude more than two layers as long as the configuration according tothe present invention is provided. Specifically, at least two of theselayers make exchange coupling, and also, the magnetic layers areselected so that the residual area magnetization increases as a resultof being heated.

A second embodiment of the present invention is described next. FIG. 7shows an elevational sectional view of a vertical magnetic recordingmedium according to the second embodiment of the present invention. Inthe figure, the same reference numerals are given to parts correspondingto those described above, and duplicated description is omitted.

As shown, the vertical magnetic recording medium 40 according to thesecond embodiment of the present invention has a configuration in which,on a substrate 21, a soft magnetic backed layer 41, a seed layer 42, anon-magnetic intermediate layer 43, a first vertical magnetic film 44, anon-magnetic coupling layer 28, a second vertical magnetic film 45, aprotective layer 30 and a lubrication layer 31, laminated in the statedorder.

The soft magnetic backed layer 41 has a thickness of, for example, in arange between 50 nm and 2 μm, and is made from an amorphous or microcrystalline alloy containing at least one element selected from amongFe, Co, Ni, Al, Si, Ta, Ti, Zr, Hf, V, Nb, C and B, or laminated filmsof these alloys. In terms of enabling concentration of a recordingmagnetic field, it is preferable to apply soft magnetic material havingmore than 1.0 T of saturation magnetic flux density. For example, FeSi,FeAlSi, FeTaC, CoNbZr, CoCrNb, NiFeNb or such may be employed. The softmagnetic backed layer 41 is produced by a plating method, a sputteringmethod, a deposition method, a CVD (chemical vapor deposition) method,or such. The soft magnetic backed layer 41 is used to absorb almost allthe magnetic fluxes from a recording head, and it is preferable that avalue of product between the saturation magnetic flux density Bs and thefilm thickness is larger in terms of carrying out saturation recording.Further, it is preferable that the soft magnetic backed layer 41 has anincreased high-frequency magnetic permeability in terms of enablingwriting at a high transfer rate.

The seed layer 42 has a thickness of, for example in a range between 1.0nm and 10 nm, and material thereof is selected from among Ta, C, Mo, Ti,W, Re, Os, Hf, Mg and an alloy thereof. Thereby, it is possible toimprove crystallinity of the non-magnetic intermediate layer 43 formedthereon and also, to break a relationship of crystal orientation andcrystal growth between the soft magnetic backed layer 41 and thenon-magnetic intermediate layer 43. This seed layer 42 may be providedor may not be provided

The non-magnetic intermediate layer 43 has a thickness of, for example,in a range between 2 nm and 30 nm, is made from non-magnetic materialsuch as Co, Cr, Ru, Re, Ri, Hf or an alloy thereof. For example, an Rufilm, an RuCo film, a CoCr film or such may be applied thereto, and itis preferable that the non-magnetic intermediate layer 43 has an hcpstructure. As a result, in a case where the first vertical magnetic film44 and the second vertical magnetic film 45 has hcp structures,epitaxial growth thereof is enabled, and crystallinity can be improved.

The first vertical magnetic film 44 and the second vertical magneticfilm 45 are so-called vertical magnetic films having a magnetizationeasy axis lying in the thickness direction, each has a thickness in arange between 3 nm and 30 nm, and each is made from any materialselected from among a group of a Co alloy, a Ni alloy, a Fe alloy, or aCo alloy containing CoPt, CoCrTa, CoCrPt, CoPt-M or CoCrPt-M, where Mdenotes one selected from among B, Mo, Nb, Ta, W and Cu. Especially, itis preferable to employ material obtained from adding, to theabove-mentioned alloy, at least one element from among rare-earthelements including Gd, Tb, Dy, Pr, Nd, Yb, Sm, Ho and Er. As describedabove for the first embodiment, in these alloys, polycrystal in whichcrystal grains are separated by crystal boundaries is produced, spinswhich Co atoms and, for example, Gd atoms of the crystal grains have aferrimagnetic arrangement, and thus, become anti-parallel to oneanother. By controlling an adding amount of these rare earth elements,it is possible to control the compensation temperature or the Curietemperature. In such a ferromagnetic alloy, crystal grains have columnarstructures in a perpendicular direction with respect to the substrateplane (thickness direction), a growth direction lies in a (001) plane ina case of the hcp structure, and the magnetic easy axis occurs in thethickness direction (such a film being simply referred to as ‘verticalcontinuous film’ hereinafter).

The first vertical magnetic film 44 and the second vertical magneticfilm 45 couple with one another antiferromagnetically in a manner ofexchange coupling by means of the non-magnetic coupling layer 28produced therebetween. The non-magnetic coupling layer 28 has athickness in a range between 0.2 nm and 1.5 nm (preferably, in a rangebetween 0.2 nm and 0.5 nm) in a case of Ru, or, in a range between 0.2nm and 1.5 nm which may vary depending on an Ru content therein in acase of Ru alloy. By setting the thickness of the non-magnetic couplinglayer 28 in this range, magnetization in the first vertical magneticfilm 44 and magnetization in the second vertical magnetic film 45 couplewith one another antiferromagnetically, and lie in anti-parallel to oneanother in a state in which no external magnetic field is applied.Especially, the thickness of the non-magnetic coupling layer 28 ispreferably determined so as to correspond to the first antiferromagneticpeak (a peak on the thinnest film thickness side) of vibration-typeexchange coupling depending on this thickness.

Relationships of the Curie temperatures and the residual areamagnetization in the first vertical magnetic film 44 and the secondvertical magnetic film 45 are set, the same as those in theabove-described first embodiment of the present invention. The materialsare selected so that the net residual area magnetization |Mr1×t1−Mr2×t2|increases when they are heated from around the room temperature, whereMr1 and Mr2 denote respective ones of residual magnetization in thefirst vertical magnetic film 44 and residual magnetization in the secondvertical magnetic film 45, and t1 and t2 denote respective filmthicknesses.

Further, it is preferable to perform setting such that Mr1×t1<Mr2×t2.Thereby, the second vertical magnetic film 45 has magnetization in thesame direction as that of the net residual area magnetization,information can be accurately recorded in the second vertical magneticfilm 45 corresponding to a position at which a recording magnetic fieldof a magnetic head is inverted, a width of magnetization transition zonecan be narrowed, and, since the second vertical magnetic film 45 whichcarries magnetic leakage field at a time of reproduction is near to themagnetic head, resolution is improved.

At least one of the first and second vertical magnetic films 44 and 45may contain non-magnetic material including a compound of at least anyone element from among Si, Al, Ta, Zr, Y and Mg, with at least oneelement from among O, C and N, and may have a non-magnetic phase whichphysically separates a crystal grain having a columnar structure in theabove-mentioned ferromagnetic alloy from an adjacent crystal grain (sucha structure being simply referred to as a ‘columnar granular structure’,hereinafter). For example, (CoPt)—(SiO₂), (CoCrPt)—(SiO₂),(CoCrPtB)—(MgO) or such may be employed. Since the magnetic grain form acolumnar structure and the non-magnetic phase is produced to surroundthe magnetic grain, the magnetic gains are separated from each other,thus, interaction between the magnetic gains is effectively reduced orbroken, and thus, it is possible to reduce medium noise.

In a case where the columnar granular structure is applied to any one ofthe first vertical magnetic film 44 and the second vertical magneticfilm 45, the above-described vertical continuous film may be employed asthe other one. For example, by employing (CoCrTa)—(SiO₂) of the columnargranular structure as the first vertical magnetic film 44 whileemploying the vertical continuous film of CoCrPtB as the second verticalmagnetic film 45, it is possible to control a reduction rate of residualmagnetization occurring due to temperature rise, also as a result ofselecting the size and separation of the crystal grains. Thereby, it ispossible to increase the net area magnetization |Mr1×t1−Mr2×t2| at atime of heating.

Furthermore, at least one of the first and second vertical magneticfilms 44 and 45 may be made of an artificial lattice film of Co/Pd,CoB/Pd, Co/Pt, CoB/Pt or such. The artificial lattice film is produced,for example, as a result of CoB (with a thickness of 0.3 nm)/Pd (with athickness of 0.8 nm) being alternately laminated to form finally fivelayers through 30 layers. Such an artificial lattice film has a largevertical magnetic anisotropy, and thereby, provides superior thermalstability.

In the second embodiment, temperature change is made different betweenthe first vertical magnetic film 44 and the second vertical magneticfilm 45 making antiferromagnetically exchange coupling therebetween, andthe vertical magnetic recording medium is heated. Thereby, it ispossible to obtain the net residual area magnetization larger that thataround the room temperature, and thus, it is possible to increasereproduction output. Accordingly, it is possible to increase the S/Nratio from that in the conventional vertical magnetic recording medium.Further, around the room temperature, the net residual areamagnetization can be reduced, thereby the demagnetizing field can becontrolled at this time, and, as a result, it is possible to improve thethermal stability in written bits.

A third embodiment of the present invention is described next. FIG. 8shows an elevational sectional view of a patterned medium according tothe third embodiment of the present invention. The same referencenumerals are given to parts corresponding to those described above, andduplicated description is omitted.

As shown in FIG. 8, the patterned medium 50 includes a substrate 21,laminated members 51 cyclically arranged on the substrate 21, andnon-magnetic parts 52 separating these laminated members 51 from eachother. Each of the laminate members 51 has the same configuration asthat of the in-plane magnetic recording medium according to the firstembodiment or the vertical magnetic recording medium according to thesecond embodiment described above. Here, the in-plane magnetic recordingmedium is used as an example. For the purpose of simplification ofdescription, only the first magnetic layer 28, the non-magnetic couplinglayer 28 and the second magnetic layer 29 thereof are shown, and theother parts are omitted.

The patterned medium 50 has a configuration such that the laminatedmembers 51 in each of which the first magnetic layer 26 and the secondmagnetic layer 29 make antiferromagnetically couple with one another arearranged cyclically with the non-magnetic parts 52 separatingtherebetween. A size of each of the laminated members 51 is, forexample, 30 nm×30 nm, and each non-magnetic part 52 has a width of, forexample, 10 nm. Since the laminated members 51 are separated from eachother by the non-magnetic parts 52, it is possible to reduce interactionbetween adjacent ones of the laminated members 51 so that medium noisecan be reduced.

However, when a demand for a higher recording density may result inreduction in the separation between adjacent laminated members 51 whichthereby approach each other much, magnetostatic interaction therebetweenbecomes enlarged. However, as described for the first embodiment above,according to the present invention, in the patterned medium 50, sincethe net residual area magnetization is reduced between the first andsecond magnetic layers 26 and 29 around the room temperature, a magneticfield leaking from the laminated members 51 are reduced, and thus, it ispossible to control demagnetizing fields applied to adjacent laminatedmembers 51. Accordingly, it is possible to control the magnetostaticinteraction, thereby the thermal stability can be improved and also themedium noise can be reduced.

In order to produce the patterned medium 50 according to the thirdembodiment, after an in-plane magnetic recording medium according to thefirst embodiment is formed, a silicon oxide (not shown) is formed on thesurface of the second magnetic layer 29 thereof, then, further thereon,a resist film (not shown) is formed, and grinding therein is carried outup to the first magnetic layer 26 by a photolithography method or an RIEmethod. After that, the non-magnetic parts 52 are produced by filing thethus-formed grooves with silicon oxide, diamond-like carbon or such.Finally, the silicon oxide or such on the surface thus formed isplanarized. In the photolithography method, an electron beam drawingmethod may be applied for example. Thereby, it becomes possible to carryout microscopic drawing on the order of tens of nanometers.

FIG. 9 shows a patterned medium 60 according to a first variantembodiment of the third embodiment of the present invention describedabove. In the patterned medium 60, a first magnetic layer 62 of eachlaminated member 61 includes nanoparticles 63 of ferromagnetic materialarranged in a self assembling manner. The nanoparticles 63 make exchangecoupling with a second magnetic layer 29 antiferromagnetically via anon-magnetic coupling layer 28. Temperature characteristics ofmagnetization in the ferromagnetic material of the nanoparticles 63 areadjusted by means of the ferromagnetic material which is a Co alloy, aNi alloy, a Fe alloy or such. Specifically, the temperaturecharacteristics of the residual magnetization can be controlled byadjacent of a particle size of the nanoparticles 63, separation of thenanoparticles 63, a degree of magnetic coupling between thenanoparticles 63, composition thereof, or, phase transition temperaturesuch as a Curie temperature. By thus utilizing the nanoparticles 63, itis possible to easily perform control of the characteristic values.Thereby, it is possible to improve the accuracy of controlling thetemperature characteristics. Further, although an example of a singlelayer of nanoparticles 63 is produced is shown in FIG. 9, a plurality ofthese layers may be provided. Also in such a case, since adjacentnanoparticles 63 make ferromagnetically exchange coupling with eachother, the first magnetic layer 62 made of these nanoparticles 63 makesantiferromagnetically exchange coupling with the second magnetic layer29.

Further, as shown in FIG. 10, which shows a second variant embodiment ofthe third embodiment of the present invention, in each laminated member71, a first magnetic layer 72 may employ crystal grains 73 each of whichis a microscopic in-plane continuous film or vertical continuous film,or may employ a columnar granular film described above for the secondembodiment. In such a case, the Curie temperature is appropriatelycontrolled by means of a grain size, material or such of the crystalgains.

A method of producing the patterned medium is not limited to thatdescribed above and another well-known method may be applied. Forexample, the above-mentioned laminated members may be embedded incavities cyclically produced in a substrate 21. Alternatively, asubstrate of a servo built-in type, a substrate of a land and groovetype or such may be employed instead.

A fourth embodiment of the present invention is described next. FIG. 11generally shows a magnetic storage according to the fourth embodiment ofthe present invention.

As shown, the magnetic storage 80 according to the embodiment includes ahousing 81. In the housing 81, a hub 82 driven by a spindle (not shown),a magnetic recording medium 83 fixed to the hub 82 and rotated thereby,an actuator unit 84, an arm 85 and a head suspension 86 mounted on theactuator unit 84 and moved in a radial direction of the magneticrecording medium 83, and a head slider 88 supported on the headsuspension 86. To the head slider 88, a laser irradiation optical system87 for applying a laser beam so as to heat the magnetic recording medium83 is connected. The laser irradiation optical system 87 includes alaser light source such as a semiconductor laser, an optical fiber, aconverging optical system or such for leading the laser beam to the headslider 88.

FIG. 12 shows an elevational sectional view of a part of the magneticstorage shown in FIG. 11. The same reference numerals are given to partscorresponding to those described above, and duplicated description isomitted.

As shown in FIG. 12, the head slider 88 includes a recording device 89,a GMR reproduction device 90 and a converging lens 91. As the recordingdevice 89, a ring-shaped thin-film induction type recording device,i.e., a single magnetic pole head is employed in a case where themagnetic recording medium is a vertical magnetic recording medium.Further, as the GMR reproduction device 80, a well-known one may beemployed. Instead of the GMR reduction device, a TMR (ferromagnetictunnel junction magneto resistive) device, a ballistic MR device or suchmay be employed. As the magnetic recording medium 83, the in-planemagnetic recording medium according to the first embodiment is applied,for example.

A recording process in this magnetic storage 80 is described next. Inthe magnetic storage 80, the head slider 88 floating on the magneticrecording medium 83 moving in an arrow A applies a laser beam (forexample, with a wavelength of 685 nm) so as to heat the surface of themagnetic recording medium 83 in particular at a track thereof to recordby means of the laser irradiation optical system 87, and thus performsrecording desired information thereon with the recording device 89applying a recording magnetic field. A spot diameter of the laser beam(a diameter at which a relative intensity become 50% of a peakintensity) is set as being in a range between 0.1 and 20 times the trackwidth, to record, for example. The spot diameter of the laser beam ispreferably in a range between 5 and 10 times the track width in terms ofeasiness of realizing it technically. In terms of thermal influence on atrack adjacent to the track to record on, a range between 1 and 5 timesthe track width is preferable. In the magnetic storage according to thepresent invention embodiment, even when the spot diameter of the laserbeam is set larger than the track width, the track width becomes equalto a core width of the recording device 89. In other words, the spotdiameter of the laser beam may be set larger than the core width of therecording device 89. However, the spot diameter may be set smaller thanthe core width of the recording device 89 so as to utilize thermaldiffusion to from a track width approximately equal to the core width ofthe recording device 89.

Output of the laser beam is approximately determined depending oncomposition of the first magnetic layer 26 and the second magnetic layer29, the predetermined spot diameter or such. For example, it is set in arange between 0.1 mW and 20 mW, preferably, in a range between 1 mW and5 mW.

As a result of the laser beam being applied, the temperature in thefirst magnetic layer 26 and the second magnetic layer 29 increases to100 through 200° C., for example, thereby, a magnetocrystallineanisotropy constant Ku or an anisotropic magnetic field decreases. As aresult, even when the composition is such as that having largemagnetocrystalline anisotropy constant Ku or anisotropic magnetic fieldaround the room temperature, recording can be carried out easily evenwithout increasing the recording magnetic field of the recording device89 in the above-mentioned condition in which this factors decrease.Accordingly, it is possible to keep desired overwrite performance andbit error rate easily. Therefore, it is possible to employ materialhaving a large magnetocrystalline anisotropy constant Ku as the firstmagnetic layer 26 and/or the second magnetic layer 29, for which it isnot possible to carry out recording due to deterioration of theoverwrite performance in the conventional recording method in which theheating is not performed. For example, in a case where the firstmagnetic layer and/or the second magnetic layer 29 is made from a CoCrPtalloy, it is preferable to set a Pt amount in a range between 12 atomic% and 35 atomic %. As a result, it is possible to increase a thermalstability index expressed by KuV/k_(B)T, and thus, to improve thethermal stability. Especially, it is preferable to employ materialhaving larger magnetocrystalline anisotropy constant Ku or anisotropicmagnetic field as material of the second magnetic layer 29 than that ofthe first magnetic layer 26. In fact, since the second magnetic layer 29located nearer to the head slider 88 has a large recording magneticfield applied thereto than that applied to the first magnetic layer 26,it is possible to efficiently increase the thermal stability with alower recording magnetic field as a result of material having a largermagnetocrystalline anisotropy constant Ku or anisotropic magnetic fieldbeing used as material of the second magnetic layer 29.

Furthermore, in addition to increasing the magnetocrystalline anisotropyconstant Ku of the first magnetic layer 26 and/or the second magneticlayer 29, a saturation magnetization Ms may be increased so thatincrease in the anisotropy magnetic field Hk (=2Ku/Ms) may be reduced.In fact, since the recording magnetic field in the recording device 89required to carry out saturation recording is approximately inproportion to the anisotropy magnetic field Hk, it is possible to reducethe recording magnetic field and the laser output. Specifically, in acase where the first magnetic layer 26 and/or the second magnetic layer29 is made from a CoCrPt alloy, a Pt amount and a Co amount should beincreased on the atomic concentration basis, and the elements other thanPt and Co should be reduced. Further, in case where the first magneticlayer 26 and/or the second magnetic layer 29 is made from polycrystalmaterial including crystal grains of a CoCrPt alloy, thermal treatmentmay be carried out, Cr segregation may be promoted, and Co concentrationin the crystal grains may be increased.

Further, it is preferable to set KuV/k_(B)T of the magnetic recordingmedium 83 in a heated state at an area having the laser beam appliedthereto as being more than 15 (further preferably more than 45). At thearea having the laser beam applied thereto, a portion to which therecording magnetic filed is applied by the recording device 89 hasmagnetization inversion or such occurring therein and thus newinformation is recorded, while thermal stability of residualmagnetization at a portion at which no recording magnetic field isapplied, especially, thermal stability of the residual magnetization inthe second magnetic layer 29 increases. Normally, in a hard disk drivefor which a strict thermal stability requirement is applied, anallowable value is approximately 10% reduction in residual magnetizationfor ten years in a case where it is stored in the room temperature.KuV/k_(B)T=15, mentioned above, is determined from consideration ofreduction in the residual magnetization occurring when laser beamapplication is performed ten thousand times, assuming that each time oflaser beam application is carried out for 10 ns.

Furthermore, KuV/k_(B)T of the magnetic recording medium 83 at a heatedcondition in an area to which the laser beam is applied is preferablybelow 80 in a case where the first magnetic layer 26 or the secondmagnetic layer 29 is of a continuous film, in terms of influence due tothe heating such as thermal deformation or such of the substrate 21 inthe recording medium, or a saturation magnetic field density of magneticpole material which is technically available at the present time for therecording device 89. The above-mentioned case where the first magneticlayer 26 or the second magnetic layer 29 is of a continuous filmincludes, as long as it is the continuous film, not only a case of thefirst magnetic layer 26 or the second magnetic layer 29, but also a caseof the first vertical magnetic film or the second vertical magnetic filmaccording to the second embodiment, a case of the first magnetic layeror the second magnetic layer according to the third embodiment, or acase of a magnetic layer of a magnetic recording medium having a singlemagnetic layer. In a case of a magnetic layer having magnetic grainshaving columnar granular structures or a patterned medium in whichmagnetic grains are regularly disposed in a non-magnetic substrate, theabove-mentioned KuV/k_(B)T of the magnetic recording medium 83 at aheated condition in an area to which the laser beam is preferably setless than 1500, and more preferably, less than 300. The above-mentionedcontinuous film means a film in which a non-magnetic region is formedaround a magnetic grain thanks to segregation of crystalline magneticgrains or such.

Thus, KuV/k_(B)T of the magnetic recording medium 83 at a heatedcondition in an area to which the laser beam is applied is preferably ina range between 15 and 80 in a case where the first magnetic layer 26 orthe second magnetic layer 29 is a continuous film. In a case of amagnetic layer having magnetic grains in columnar granular structures,or a patterned medium in which magnetic grains are regularly disposed ina non-magnetic substrate, the same is preferably in a range between 15and 1500.

In order to control KuV/k_(B)T as described above, the above-describedmagnetocrystalline anisotropy constant Ku of the magnetic recordingmedium or the laser output is appropriately selected. KuV/k_(B)T isobtained from, based on a formula (I) described later, measuring aso-called dynamic coercive force Hc′ for various ones of a magneticfield switching interval ‘t’. Measurement of the dynamic coercive forceHc′ is carried out with the use of an inverse DC degaussing method forexample. In the inverse DC degaussing method, recording is carried outin a condition in which the above-mentioned temperature at the heatedcondition is set.

Further, due to the laser beam application, the amount of exchangeinteraction (exchange coupling magnetic field) between the firstmagnetic layer 26 and the second magnetic layer 29 decreases, rotationof magnetization in each of the first magnetic layer 26 and the secondmagnetic layer 29 can be carried out more easily than in a recordingprocess around the room temperature, and thus, the overwrite performanceis further improved.

The temperature to which the first and second magnetic layers 26 and 29is heated due to the laser beam application is not limited to the 100°C., and any other temperature may be applied as long as it is higherthan the temperature at which the magnetic recording medium 83 isnormally used. In terms of thermal durability of the substrate, thisheating temperature is preferably selected from a range below 400° C. Interms of crystallization in a case where the amorphous layer is employedas material of the foundation layer of the magnetic recording medium 83,a range below 200° C. is especially preferable, and a range below 150°C. is further preferable. In terms of thermal stability of the firstmagnetic layer 26 and/or the second magnetic layer 29, a range higherthan 65° C. is preferable.

Further, the laser beam output, i.e., irradiation (laser application)energy applied to the magnetic recording medium 83 may be changeddepending on a recording frequency. In other words, in a case of a highfrequency, a high laser output may be applied, while in a case of a lowfrequency, a low output of laser beam may be applied. For example, for amagnetic disk 1 in a specific example described later, the laser outputof 1 mW is applied for a recording frequency of 105 kHz, while 3 mW isapplied for the recording frequency is 73 MHz, therebetween the laseroutput being set in proportion to the recording frequency. By applyingsuch a setting, it is possible to achieve high resolution and also, itis possible to achieve power saving.

A reproduction process is described next. As shown in FIG. 12, in themagnetic storage 80, the head slider floating above the magneticrecording medium 83 moving in the arrow X direction applies a laser beamso as to heat the surface of the magnetic recording medium 83 inparticular at a track thereof to reproduce information therefrom, andreproduces bits recorded in the magnetic recording medium 83 with theuse of the GMR reproduction device 90. The first and second magneticlayers 26 and 29 are heated to a predetermined temperature, andmagnetization M1 a in the first magnetic layer 26 changes intomagnetization M1 b smaller in residual magnetization than that ofmagnetization M1 a. On the other hand, similarly, magnetization M2 a inthe second magnetic layer 29 changes into magnetization M2 b. However,since a reduction rate in the residual magnetization of the firstmagnetic layer 26 is larger than that of the second magnetic layer 29,and also, the film thickness is set approximately equal between thefirst and second magnetic layers 26 and 29, the net residual areamagnetization thereof becomes larger in a heated state than that in anot heated state. Therefore, reproduction output in proportion to thenet residual area magnetization thus increased thanks to the heating isobtained from the GMR reproduction element 90, and thus, thereproduction output increases and the S/N ratio increases.

A heating temperature at the reproduction process is set at atemperature at which the net residual area magnetization increasesthanks to selection of the materiel, composition or such of the firstmagnetic layer 26 and/or the second magnetic layer 29, higher than atemperature at which the magnetic recording medium 83 is normally used.This heating temperature is selected from a temperature range in whichboth the residual magnetization in the first magnetic layer 26 and theresidual magnetization in the second magnetic layer 29 do not vanish. Itis preferable to select this temperature within this range and also, itis preferable to select from a range blow 400° C. in terms of heatdurability of the substrate. Further, it is preferable to select from arange below 200° C. in terms of crystallization in a case where theamorphous layer is employed as material of the foundation layer of themagnetic recording medium 83, and a range below 150° C. is furtherpreferable. Furthermore, in terms of thermal stability of the firstmagnetic layer 26 and/or the second magnetic layer 29, a range higherthan 65° C. is preferable.

The magnetic recording material 82 is not limited to the in-planemagnetic recording medium according to the first embodiment.Alternatively, the vertical magnetic recording medium according to thesecond embodiment or the patterned medium according to the thirdembodiment may be employed. In the magnetic storage applying therecording process according to the present embodiment, the magneticrecording medium 83 is not limited to the magnetic recording medium inany one of the first through the third embodiments in which the netresidual area magnetization increases as a result of the recordingmedium being heated. That is, as materials of the first and secondmagnetic layers 26 and 29, materials having approximately sametemperature characteristics for residual magnetization, i.e., havingsimilar compositions, for example, may also be employed. Further, awell-known synthetic ferrimagnetic material may also be employed. Forexample, the magnetic recording medium disclosed as an embodiment inJapanese Laid-open Patent Application No. 2001-056924 may be employed.

Furthermore, instead of the first magnetic layer/the non-magneticcoupling layer/the second magnetic layer according to the firstembodiment or the first vertical magnetic film/the non-magnetic couplinglayer/the second vertical magnetic film according to the secondembodiment, a magnetic layer of a single layer having an in-planeorientation or a magnetic recording medium having a vertical magnetizedfilm may be employed. Other than these, a magnetic recording mediumhaving a magnetic layer with a magnetization orientation direction ofapproximately 45 degrees from a substrate surface, a magnetic layer witha magnetization orientation direction of larger than 0 degrees andsmaller than 45 degrees from a substrate surface may also be employed.

Furthermore, the magnetic grains in the first magnetic layer or thesecond magnetic layer according to the first embodiment, the firstvertical magnetic film or the second vertical magnetic film according tothe second embodiment, the first magnetic layer or the second magneticlayer according to the third embodiment, the nanoparticles, the magneticlayer in the magnetic recording medium having the single magnetic layer,or the patterned medium in which the magnetic grains are regularlydisposed in the non-magnetic substrate, is preferably made from CoPt, orfrom material having CoPt as a chief ingredient as well as B, Mo, Nb,Ta, W, Cu, Cr or an alloy thereof being added thereto. Furthermore, a Ptamount of these materials is in particular preferably in a range between12 atomic % and 35 atomic % in a case where the magnetic layer is acontinuous film. In a case where the magnetic layer is made frommagnetic grains having columnar granular structures, nanoparticles, ormagnetic particles of the patterned medium in which the magnetic grainsare disposed regularly in the non-magnetic substrate, a setting in arange between 1 atomic % and 35 atomic % is especially preferable(further preferably, in a range between 1 atomic % and 25 atomic % interms of achieving stability of hcp (hexagonal closest packing)structure). As a result, it is possible to increase the index of thermalstability expressed by KuV/k_(B)T, and improve the thermal stability.The above-mentioned setting is preferable in the case of the patternedmedium in which magnetic grains are regularly disposed in thenon-magnetic substrate, since, by increasing the magnetocrystallineanisotropy constant Ku and also increasing the saturation magnetizationMs of the magnetic grains, both the anisotropic magnetic field Hk andincrease in the medium noise can be controlled easily.

Although the head slider 88 is of a lens integrated type in which therecording device 89, the GMR reproduction device and the converging lens91 are provided integrally, the converging lens 91 and the laserirradiation optical system 87 may be provided separately from the headslider. Instead of the converging lens 91 and the laser irradiationoptical system 87, any other device may be employed as long as it has afunction of selectively heating a track on the magnetic recording mediumto record thereto or reproduce therefrom.

The specific example of the above-described fourth embodiment of thepresent invention is described next. As the magnetic recording medium, amagnetic disk having the following configuration was used. In themagnetic disks 1 and 2, the composition of the second magnetic layer wasmade different therebetween, while a configuration from the glasssubstrate through the non-magnetic coupling layer was made in common.The configuration was produced including the glass substrate/CrTi layer(with a thickness of 25 nm)/AlRu layer (with a thickness of 15 nm)/CrMolayer (with a thickness of 5 nm)/CoCrTa layer (with a thickness of 1nm)/CoCr layer (with a thinness of 1.5 nm)/Ru layer (with a thickness of0.7 nm). Furthermore, on the second magnetic layer, a DLC film (with athickness of 4 nm)/lubrication layer (with a thickness of 1.5 nm) wereproduced. The magnetic disks 1 and 2 having the second magnetic layersof the following materials, respectively, were produced:

the magnetic disk 1: CoCrPt₁₄B layer (with a thickness of 15 nm); and

the magnetic disk 2: CoCrPt₁₇B layer (with a thickness of 15 nm).

In each of the magnetic disks 1 and 2, in a DC magnetron sputteringmethod, a substrate temperature was set at 240° C. or less, the CrTilayer through the DLC layer were produced, the lubrication layer wascoated in a puling-up method, and after the coating, heating treatmentwas carried out on the lubrication layer at 110° C. for one hour in theatmosphere with the use of an oven.

FIG. 13 shows magnetic characteristics of the magnetic disks 1 and 2. Asshown, because of a difference in the Pt amount, the magnetic disk 2 haslarger coercive force and saturation magnetic flux density than those ofthe magnetic disk 1 at 25° C. Further, respective magnetocrystallineanisotropy constants at 0 K of the magnetic disks 1 and 2 obtained fromthe temperature characteristics of the saturation magnetic flux densityand the coercive force in a range between 10 K and 300 K measured by theSQID apparatus are such that the value is larger in the magnetic disk 2than that in the magnetic disk 1 by approximately 20%.

On the other hand, a so-called dynamic coercive force Hc′, which is acoercive force of the magnetic dick when a recording magnetic field isswitched at a high speed is expressed by the following expression (1)according to Bertram (H. N. Bertram, H. J. Richter, Arrhenius-Neel: J.Appl. Phys. vol. 83, No. 8, pp. 4991 (1999)):Hc′=0.474Hk{1−1.55 [(k _(B) T/KuV)×ln(fot/ln 2)/2]}^(2/3)  (1)There, Hk denotes an anisotropic magnetic field, k_(B) denotes theBotzmann constant, T denotes a temperature, Ku denotes amagnetocrystalline anisotropy constant, V denotes a volume of bits ofexchange coupling made between the first and second magnetic layers 26and 29, ‘fo’ denotes an attempt frequency, and ‘t’ denotes a magneticfield switching interval. According to the expression (1), the dynamiccoercive force Hc′=0.474 Hk, at T=0 K, and, since the dynamic coerciveforce does not depend on the magnetic field switching interval t, itbecomes equal to the coercive force. The coercive force at 0 K obtainedfrom the temperature characteristics in a range between 0 K and 300 Kis, as shown in FIG. 13, approximately equal between the magnetic disk 1and the magnetic disk 2. Therefrom, it can be seen that also theanisotropic magnetic field Hk is approximately equal between themagnetic disks 1 and 2.

As can be seen from the above, in the magnetic disk 2, with respect tothe magnetic disk 1, the magnetocrystalline anisotropy constant Ku andthe saturation magnetic flux density were increased without much changein the anisotropic magnetic field Hk, and thus, improvement in thethermal stability was attempted. With reference to FIGS. 14 and 17,specific description therefor is made next.

FIG. 14 shows the thermal stabilities of the magnetic disks 1 and 2. Theordinate denotes a residual magnetization reduction rate, where theresidual magnetization reduction rate becomes larger in the downwarddirection. A unit ‘%/decade’ of the residual magnetization reductionrate is expressed by (1−M2/M1)×100, where M1 denotes reference residualmagnetization at a time t from beginning of applying a demagnetizingmagnetic field, and M2 denotes residual magnetization at a time 10t(=ten times the time t). The abscissa denotes the demagnetizing magneticfield Hd. Measurement of the thermal stability was carried out asfollows: Chips (each having a size of approximately 7 mm×7 mm) were cutout from the magnetic disks 1 and 2, respectively, then they weremagnetized in one direction, and, after that, while a demagnetizationmagnetic field Hd selected from a range between 0 and −1500 Oe (118.5kA/m) was applied in the opposite direction, the residual magnetizationreduction rate at 27° C. (300 K) was measured by the SQID apparatus. Themore the residual magnetization reduction rate approximates 0, i.e., itgoes in the upward direction in the figure (graph), the more the thermalstability is improved.

As shown in FIG. 14, for example, when the demagnetizing magnetic fieldHd is −800 Oe, the residual magnetization reduction rate is reduced tobe approximately ⅙ in the magnetic disk 2 with respect to the magneticdisk 1. In other words, it can be seen that, by employing materialhaving a large magnetocrystalline anisotropy constant as material of thesecond magnetic layer, it is possible to improve the thermal stability.

FIGS. 15A and 15B show relationships between the overwrite performanceand the laser output. FIG. 15A shows the overwrite performance of themagnetic disk 1 while FIG. 15B shows the overwrite performance of themagnetic disk 2. In the figures, numeric values denote the amounts ofthe laser output.

As shown in FIGS. 15A and 15B, in a case where no laser beam is appliedat recording (in the figures, expressed by 0 mA), even when therecording current is 48 mA, for each of the magnetic disks 1 and 2, theoverwrite performance does not reach a satisfactory value of −30 dB. Onthe other hand, when a laser beam is applied, the overwrite performanceis improved as the laser output is increased, and, the overwriteperformance is improved from −30 dB at a time of 3 mW in the case of themagnetic disk 1, and at a time of 5 mW in the case of the magnetic disk2. Accordingly, it can be seen that, even for the magnetic disks 1 and2, for which recording cannot be carried out when no laser beam isapplied, i.e., according to the conventional method, and thus, theoverwrite performance is not satisfactory in this state, the overwriteperformance can be remarkably improved therefrom as a result of a laserbeam being applied.

Measurement of the overwrite performance was carried out with the use ofa spin stand for evaluating a magnetic disk (provided by KyodoElectronic System, Co. Ltd., a product name of LS90), in a conditionwhere the saturation magnetic flux density of the magnetic pole in therecording device was set as 2.4 T, the core width thereof was 0.3 μm,the core width of the GMR reproduction device was 0.19 μm, the laserbeam wavelength was 685 nm, the spot size was 1.1 μm, the recordingposition radius=25.5 mm, the rotation speed was 2000 RPM, the longwavelength was 87 kfci, and the short wavelength was 700 kfci, and, thelaser output was changed from 0 mW through 10 mW. The laser beam wasapplied from the glass substrate surface side on which no films areformed, and was focused on the second magnetic layer or such formedthereon. The other characteristics described later were also measured bythe same conditions unless otherwise stated.

FIG. 16 shows solitary wave half-value width (PW50) characteristics ofthe magnetic disks 1 and 2. The laser power was set as 3 mW for themagnetic disk 1 while the same was 5 mW for the magnetic disk 2.Further, the requirement applied was such that the overwrite performanceshown in FIGS. 15A and 15B should be ensured as −30 dB.

As shown in FIG. 16, the solitary wave half-value width PW50 isapproximately saturated for each of the magnetic disks 1 and 2 with themagnetic recording current of approximately 16 mA, and PW50 has aconstant value at least in a range between 20 mA and 40 mA in therecording current. Further, not shown, it was proved that the output issaturated with the recording current of more than 16 mA. Conventionally,the recording current is set as 40 mA. In contrast thereto, it can beseen that, the solitary wave half-value width PW50 is saturated with alower recording current according to the present invention, i.e.,resolution is satisfactory with the lower recording current, andtherefrom, it can be seen that a range of the recording current fromwhich selection should be made can be effectively widened according tothe present invention. Furthermore, by reducing the recording current,the recording magnetic field distribution from the recording devicebecomes satisfactory, and, thus, it is possible to remarkably reducegeneration of side erase or magnetic transition zone width increaseoccurring due to the recording magnetic field spreading in the in-planedirection of the recording medium from the recording device with therecording current on the order of 40 mA in the prior art. Themeasurement was carried out in a condition in which the recordingdensity of the solitary wave half-value width PW50 was set at 24 kfci.

FIG. 17 shows a relationship between the S/N ratio and the laser outputin the magnetic disk 1. The S/N ratio is expressed by a ratio between anaverage signal output S (at the recording density of 350 kfci) and amedium noise Nm. Numeric values in the figure denote recording currents.In the figure, shown is a result for the magnetic disk 1 having the filmthickness in the second magnetic layer of 19 nm.

As shown, when the recording current was 15 mA, 30 mA or 40 mA, otherthan 10 mA, the S/N ratio was maximum with the laser output in a rangebetween 2 and 4 mA. In other words, it can be seen that the S/N ratio isnot much influenced even from somewhat change in the laser output.Furthermore, it is easily expected that, even from a relatively slightlaser output, the maximum S/N value can be achieved, and thermalinfluence such as degaussing on the magnetic disk can thus be reduced.Although not shown for the magnetic disk 2, a range of the laser outputin which the S/N ratio became maximum was approximately identical alsofor the magnetic disk 2.

FIG. 18 shows a relationship between the laser output and the recordingcurrent with which the S/N ratio of the magnetic disks 1 and 2 becomesmaximum. FIG. 18 shows the relationship between the laser output and therecording current, with which the S/N ratio of the magnetic disk 1becomes maximum shown in FIG. 17, and also, shows the same for themagnetic disk 2.

As shown in FIG. 18, 10 mW in the laser output is required to maximizethe S/N ratio with the recording current in the recording device of 10mA for the magnetic disk 1, while with the recording current of 15 mA,the laser output required is reduced to 3 mW, and also, the laser outputrequired to maximize the S/N ratio is approximately fixed at 3 mW evenwhen the recording current is further increased. For the magnetic disk2, the recording current from which the laser output required tomaximize the S/N ratio becomes approximately fixed is 16 mA. That is, inthe magnetic storage in the prior art in which no laser beam is applied,normally, the recording current is set on the order of 50 mA for thepurpose of ensuring the predetermined overwrite performance or such. Incontrast thereto, in the embodiment according to the present invention,it can be seen that even 16 mA which is less than half of theabove-mentioned 50 mA in the recording current can maximize the S/Nratio. The recording current from which the laser output maximizing theS/N ratio becomes approximately fixed corresponds to, as describedbelow, the recording current (referred to as a ‘magnetic fieldsaturation recording current’ hereinafter) at which the inclination ofthe generated magnetic field of the recording device with respect to therecording current remarkably decreases.

FIG. 19 shows a relationship between the generated magnetic field in therecording device and the recording current. The relationship in FIG. 19is a relationship between the generated magnetic field (maximum value)and the recording current around a recording gap in the recording deviceobtained by calculation.

As shown in FIG. 19, it can be seen that the generated magnetic field inthe recording device sharply increases in response to increase in therecording current from 10 mA, a degree of the increase, i.e., aninclination of the generated magnetic field then starts decreases around15 mA of the recording current (magnetic field saturation recordingcurrent), and, then, the increase in the generated magnetic field issmall even when the recording current is further increased.

As mentioned above, in the conventional magnetic storage in which nolaser beam is applied, normally, the recording current is set on theorder of 50 mA for the purpose of ensuring the overwrite performance orsuch. In this case, after the magnetic field saturation recordingcurrent is exceeded, the spatial distribution of the generated magneticfield spreads so that a concentration degree of the generated magneticfield deteriorates. As a result, at an extending end of the recordingdevice, a magnetic field leaks also from a portion other than a surfacefacing the magnetic disk, and, as a result, a problem may occur in whichinformation recorded in an adjacent track is erased, i.e., a side eraseproblem may occur. Also, another problem of head crash may occur due tothe core (magnetic pole) of the recording device projecting due to beingheated by the recording current, or such. In order to solve theseproblems, it is preferable that the recording current should be lowwhile it should be more than 15 mA. Accordingly, it is preferable to setthe recording current in a range between 15 mA and 40 mA, and, it isfurther preferable to set the same in a range between 15 mA and 30 mA.

The relationship shown in FIG. 19 was obtained by calculation asmentioned above, and, as described below, an output saturation currentIsat usable instead of the magnetic field saturation recording currentcan be easily obtained from measurement.

FIG. 20 shows a relationship between an average output in a lowrecording density and a recording current in the magnetic disk 1. Inthis case, the measurement was carried out, in a condition in which, ofthe above-mentioned measurement condition for the present example, therecording density was set at 24 kfci, and the laser output was set at 1mW.

With reference to FIG. 20, how to obtain the output saturation currentIsat is described next. First, the maximum value Vmax of an averageoutput is obtained. After that, the recording current Io (12.4 mA) withwhich 90% of Vmax (0.9 Vmax in FIG. 20) is obtained, is obtained. Then,the recording current which is 150% thereof (=1.5×Io) is obtained as theoutput saturation current Isat. As shown in FIG. 20, the thus-obtainedoutput saturation current Isat is 18.6 mA, and, it can be seen that therecording current thus obtained is that, with which the average outputis saturated. In the above-described relationship between the magneticfield saturation recording current and the output saturation currentIsat, since the magnetic field saturation recording current is 15 mA asmentioned above, it can be approximately said that, the magnetic fieldsaturation recording current=output saturation current Isat×80%. Infact, 18.6×0.8≈15. Accordingly, in terms of the above-mentioned problemsof the spatial distribution of the generated magnetic field, the sideerase, and the projection of the magnetic pole, it is preferable to setthe recording current in a range between 80% and 215% of the outputsaturation current Isat, and, it is further preferable to set the samein a range between 80% and 160%.

Next, influence of laser beam application at a time of recording oninformation already recorded, i.e., influence on the residualmagnetization is described.

FIG. 21 shows a change in a normalized average output at a time of laserbeam application to the magnetic disk 1. In the figure, the ordinatedenotes a normalized average output obtained from normalizing an averageoutput with an initial average output measured after recording at arecording density of 350 kfci. The abscissa denotes laser applicationenergy applied when the laser output is changed while the laser beamspot is approximately fixed. Measurement was carried out as follows: (1)Recording was carried out at a line density of 350 kfci; (2) An averageoutput (i.e., the initial average output) was measured; (3) Laser beamapplication with predetermined laser beam application energy was carriedout for one round of a track; (4) An average output (i.e., the averageoutput after the beam application) was measured; and (5) The normalizedaverage output=(the average output after the beam application)/(initialaverage output) was calculated. Then, the above-mentioned processes (1)through (5) were carried out for different laser beam applicationenergy.

As shown in FIG. 21, no reduction occurs in the normalized averageoutput of the magnetic disk 1 from the initial average output in a rangeof the laser beam application energy between 210 J/m³ and 1050 J/m².With the laser beam application energy of 1260 J/m², reduction ofapproximately 5% occurs. Accordingly, in consideration of a possibilitythat a laser beam is applied to an area other than an area to currentlyrecord on the magnetic recording medium, i.e., a possibility that alaser beam is applied to an area for which already written informationshould be saved, it can be seen that the laser beam application energyat a time of recording should be preferably set less than 1050 J/m²within which the normalized average output does not decrease. For themagnetic disk 1, as shown in FIG. 15A, it is preferable that the laserbeam at a time of recording has the laser beam application energy morethan 360 J/m² corresponding to the laser output of 3 mW to ensure theoverwrite performance of −30 dB. Accordingly, it can be seen that thelaser beam application energy at a time of recording should bepreferably set in a range between 360 J/m³ and 1050 J/m². However, thisrange of the laser beam application energy may be changed depending onthe magnetic characteristics of a particular magnetic disk. For example,for the magnetic disk 2, the magnetocrystalline anisotropy constant Kuis larger than that of the magnetic disk 1, and thereby, the upper limitthe preferable range of the laser beam application energy is expected tobe increased.

Thus, according to the specific example of the fourth embodiment of thepresent invention, by heating the surface of the magnetic disk at a timeof recording, it is possible to achieve satisfactory overwriteperformance and resolution and to achieve the high S/N ratio, with theuse of the magnetic disk having the large magnetocrystalline anisotropyconstant and having the satisfactory thermal stability, withoutincreasing the recording magnetic field. Since it is possible to reducethe recording current from that in the case of the conventional method,it is possible to carry out recording in a satisfactory condition of therecording magnetic field distribution, so as to further improve theresolution, and also to solve the various problems such as that of theside erase or such.

Further, the present invention is not limited to the above-describedembodiments and specific examples, and variations and modifications maybe made without departing from the basic concept of the presentinvention claimed below.

For example, in the magnetic storage according to the fourth embodiment,as the magnetic recording medium, it is not necessary to limit it to themagnetic disk, but, a magnetic tape in a helical scanning type or aserpentine type, or, a form of a card may also be applied.

The present application is based on Japanese priority application No.2004-061225 and 2004-000632, filed on Mar. 4, 2004 and Jan. 5, 2004,respectively, the entire contents of which are hereby incorporated byreference.

1. A magnetic storage comprising: a magnetic recording medium having arecording layer containing crystalline magnetic grains; a heating unitheating selectively said magnetic recording medium; and a recording unithaving a magnetic recording head, wherein: said heating unit heats saidmagnetic recording medium while said magnetic head records informationto said magnetic layer, said heating unit comprises a laser irradiationoptical system applying a laser beam on a surface of said magneticrecording medium, and output of the laser beam is controlled accordingto a recording frequency at which said recording head records.
 2. Themagnetic storage as claimed in claim 1, wherein: said recording layercomprises a polycrystal comprising crystal grains, a columnar granularstructure having a crystalline grain in a columnar structure andnonmagnetic material surrounding it, or a structure in whichnanoparticles are disposed.
 3. The magnetic storage as claimed in claim1, wherein: a spot diameter on said surface of the magnetic recordingmedium produced by the laser beam lies in a range between 0.1 and 20times of a track width.
 4. The magnetic storage as claimed in claim 1,wherein: a spot diameter on said surface of the magnetic recordingmedium produced by the laser beam is larger than a core width of saidrecording head.
 5. The magnetic storage as claimed in claim 3 claim 1,wherein: the laser beam is set in a range of a laser application energyamount per unit area such that, within said range, once the laser beamis applied to an area in which information is recorded, and an averageoutput of said area before and after the beam application may beapproximately constant.
 6. The magnetic storage as claimed in claim 1,wherein: a recording current in said magnetic recording head is set in arange between 80% and 215% of an output saturation recording currentvalue at which a magnetic field generated is approximately saturatedwith respect to the recording current.
 7. The magnetic storage asclaimed in claim 1, wherein: KuV/kT of said magnetic recording medium atthe heating temperature is set in a range between 15 and 80 in a casewhere said recording layer comprises a continuous film.
 8. The magneticstorage as claimed in claim 1, wherein: said magnetic layer comprises: afirst magnetic layer; and a second magnetic layer formed on said firstmagnetic layer, wherein: said first magnetic layer and said secondmagnetic layer make exchange coupling with one another, and also,magnetization of said first magnetic layer and magnetization of saidsecond magnetic layer are in anti-parallel to one another in a conditionin which no external magnetic field is applied.
 9. The magnetic storageas claimed in claim 8, wherein: a non-magnetic coupling layer isprovided between said first magnetic layer and said second magneticlayer.
 10. The magnetic storage as claimed in claim 8, wherein: saidfirst magnetic layer and said second magnetic layer comprise in-planeorientation films or vertically magnetized films.
 11. The magneticstorage as claimed in claim 8, wherein: said first magnetic layer andsaid second magnetic layer form a patterned medium.
 12. The magneticstorage as claimed in claim 8, wherein: said first magnetic layer and/orsaid second magnetic layer comprises material selected from among agroup of CoPt, CoCrPt, CoPt-M and a Co alloy containing CoCrPt-M, whereM denotes B, Mo, Nb, Ta, W, Cu or an alloy thereof, where a Pt amount isset in a range between 12 atomic % and 35 atomic %.
 13. The magneticstorage as claimed in claim 8, wherein: said second magnetic layer has alarger anisotropic magnetic field than that of said first magneticlayer.
 14. A magnetic storage comprising: a magnetic recording mediumcomprising a first magnetic layer and a second magnetic layer formed onsaid first magnetic layer, wherein said first magnetic layer and saidsecond magnetic layer make exchange coupling with one another, and also,magnetization of said first magnetic layer and magnetization of saidsecond magnetic layer are in anti-parallel to one another in a conditionin which no external magnetic field is applied; a heating unit heatingselectively said magnetic recording medium; and a magneticrecording/reproducing unit comprising a magnetic recording head and amagnetic reproducing device, wherein: said heating unit heats saidmagnetic recording medium while said magnetic recording head recordsinformation on said magnetic recording medium, said heating unitcomprises a laser irradiation optical system applying a laser beam on asurface of said magnetic recording medium, and output of the laser beamis controlled according to a recording frequency at which said recordinghead records.
 15. A magnetic storage comprising: a magnetic recordingmedium comprising a first magnetic layer and a second magnetic layerformed on said first magnetic layer, wherein said first magnetic layerand said second magnetic layer make exchange coupling with one another,and also, magnetization of said first magnetic layer and magnetizationof said second magnetic layer are in anti-parallel to one another in acondition in which no external magnetic field is applied; a heating unitheating selectively said magnetic recording medium; and arecording/reproducing unit comprising at least a magnetic recordinghead, wherein: said heating unit heats said magnetic recording medium soas to increase reproduction output and said recording/reproducing unitreproduces recorded information from said magnetic recording medium,said heating unit comprises a laser irradiation optical system applyinga laser beam on a surface of said magnetic recording medium, and outputof the laser beam is controlled according to a recording frequency atwhich said recording head records.
 16. The magnetic storage as claimedin claim 15, wherein: said magnetic recording medium having a recordinglayer containing crystalline magnetic grains.
 17. The magnetic storageas claimed in claim 15, wherein: said magnetic recording mediumcomprises: first through n-th magnetic layers, wherein: at least two ofsaid n magnetic layers make exchange coupling therebetweenantiferromagnetically; and net residual area magnetization of said firstthrough n-th magnetic layers at a first temperature is larger than thenet residual magnetization at a second temperature lower than said firsttemperature, where n denotes an integer equal to or larger than three.18. A method for reproducing information recorded by arecording/reproducing unit having a magnetic recording head on amagnetic recording medium comprising two magnetic layers making exchangecoupling with one another, comprising the steps of heating said magneticrecording medium, and causing a residual magnetization of one of thesemagnetic layers to increase at a rate higher than that at which residualmagnetization of the other magnetic layer is thereby caused to increase;and reproducing the information recorded on said magnetic recordingmedium, wherein said heating step includes a heating unit comprising alaser irradiation optical system applying a laser beam on a surface ofsaid magnetic recording medium, and wherein output of the laser beam iscontrolled according to a recording frequency at which said recordinghead records.
 19. The method as claimed in claim 18, wherein: said twomagnetic layers make exchange coupling with one anotherantiferromagnetically.